1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly, to an image forming apparatus to prevent cross contamination by controlling a development voltage of a developing unit.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus produces a desired color image by receiving a digital image signal corresponding to the image and forming an electrostatic latent image on a photosensitive medium using an exposing unit, such as a laser scanning unit (LSU), developing the electrostatic latent image into a toner image using toner, transferring the toner image to a recording medium, and fusing the toner image onto the recording medium by applying heat and pressure thereto.
Toner colors used in a color image forming apparatus are yellow (Y), magenta (M), cyan (C), and black (K). Therefore, the color image forming apparatus requires four developing units to attach the four color toners onto an electrostatic latent image.
A color image forming apparatus can be classified as a single-pass type in which four exposing units and four photosensitive media are included, and a multi-pass type in which a single exposing unit and a single photosensitive medium are included.
The single-pass type color image forming apparatus is generally used as a high-speed image forming apparatus since the time required for color printing is the same as the time required for monochrome printing. However, the single-pass type color image forming apparatus is costly and difficult to miniaturize because the apparatus includes the four exposing units and the four photosensitive media.
The multi-pass type color image forming apparatus includes the single photosensitive medium and the single exposing unit, and forms a full color toner image on an intermediate transfer medium by repeatedly performing a light exposing process, a developing process, and a transfer process for each of yellow, magenta, cyan, and black toner color images, such that the toner images are formed on the intermediate transfer medium in an overlapping manner. Then, the multi-pass type color image forming apparatus transfers the color toner image to a sheet of paper, and fuses the color toner image to the paper. Thus, a print speed of the multi-pass type color image forming apparatus is slower than that of the single-pass type color image forming apparatus, and color alignment is difficult.
To transfer toner having a specific polarity from each developing unit to a photosensitive medium, a development voltage of several hundreds volts to several thousands volts is applied to each developing unit.
FIG. 1 is a graph illustrating an example of a development voltage periodically applied to a developing roller in a conventional non-contact developing type image forming apparatus.
Referring to FIG. 1, a toner having a negative polarity is described as an example. A photosensitive medium is charged to a surface potential Vo of −700 V by a charging device, and a portion where an image is formed (that is, a portion to which a toner is attached) is exposed by an exposing unit, and thus an electric potential at the portion is increased to an image potential VL of −100 V.
To attach the negative toner to the photosensitive medium, a development voltage Vd of −1200 V is applied to a developing roller. The negative toner is jumped to the photosensitive medium from the developing roller due to a repulsive force against the development voltage Vd.
In this case, since a potential difference between the development voltage Vd of the developing roller and the image potential VL of the photosensitive medium is larger than a potential difference between the development voltage Vd and the surface voltage Vo, the negative toner is moved to only the exposed portion charged to the surface voltage VL and attaches thereto. However, some toner having a high electrical mobility among the negative toner may be attached to a non-exposed portion that is charged to the surface voltage Vo. Since the toner attached to the non-exposed portion causes an undesired image that results in contamination, the toner has to be removed from the non-exposed portion.
Generally, in a non-contact developing type image forming apparatus, a development voltage and a collection voltage are alternately applied to a developing roller. This is because a uniform image can be obtained by a repetitive movement of toners between the developing roller and the photosensitive medium since when the development voltage is directly applied without the collection voltage, it is difficult to attach toners uniformly to the photosensitive medium.
When a collection voltage Vc of +300 V is applied to the developing roller, a negative toner attached to an exposed portion of the photosensitive medium does not move to the developing roller when a difference between the collection voltage Vc and an image potential VL is smaller than a threshold potential Vth, but a negative toner attached to a non-exposed portion of the photosensitive medium moves to the developing roller since a potential difference (1000 V) between the collection voltage Vc and a surface voltage Vo is large. Consequently, the development voltage and the collecting voltage are alternately applied to the developing roller in a predetermined duty ratio, and thus the toner does not attach to the non-exposed portion of the photosensitive medium.
In such a conventional AC voltage applying method, since the total time period t0 within which a development voltage Vd and a collection voltage Vc are applied is fixed, when the length of time that either the development voltage Vd or the collection voltage Vc is applied is increased, the length of time that the other voltage is applied is decreased, and thus a developing condition cannot be controlled.
Meanwhile, in an electric potential division developing method in which a tri-level potential is applied to a photosensitive medium and toners having a positive polarity and a negative polarity are simultaneously developed, toner can be attached to a non-image area, thus causing contamination. Specifically, when a development voltage is applied to a developing roller so that a negative polarity toner is attached to the photosensitive medium to which a positive polarity toner has already been attached, some of the positive polarity toner may move to the developing roller, or some of the negative polarity toner may attach to the positive polarity toner (having a polarity opposite to the negative polarity toner), which results in contamination.